add engines property in package.json to encourage using proper node version
add engines property in package.json to encourage using proper node version

juandak16/wp-theme-starter,tylershuster/wp-theme-starter,MozaikAgency/wp-theme-starter,champagne-randy/wp-theme-bootstrap,usine/wp-usine,MozaikAgency/wp-theme-bootstrap,MozaikAgency/wp-theme-bootstrap,juandak16/wp-theme-starter,usine/wp-usine,MozaikAgency/wp-theme-starter,champagne-randy/wp-theme-bootstrap,tylershuster/wp-theme-starter